Think back to the first scene. It was a guy with plaster on his face.
Knowing what we know of what the two brothers do; kidnap, torture, befriend, have sex, then terminate.. then we can assume they did the same thing with the man at the beginning of the film.
Either that or it was some piss poor writing/directing (which we already knew because of all the other major plot holes and slow paced movement of the movie).
the person who kills the Plaster guy...if you see the movie till end its he Girl her self who later on starts killing rapist... plaster guy is not killed by these two brothers but by the kidnapped girl in the end.